Austenitic alloys



United States Pateiit O AUSTENITIC ALLOYS Wasil W. Dyrkacz, Newtonville, Richard K. Pitler, Al-

bany, and Henry M. Butler, Watervliet, N. Y., assignors to Allegheny Ludluni Steel Corporation, Brackenridge, Pa., a corporation of Pennsylvania No Drawing. Application April 25, 1957 Serial No. 655,000

Claims. (Cl. 75-126) This invention relates to age hardenable austenitic alloys and, in particular, to age hardenable austenitic a1- loys which are exceptionally well suited for applications where high levels of strength, ductility and hardness and a moderate degree of oxidation and corrosion resistance are required at temperatures of up to about 1600 F.

V In applications such as internal combustion engine exhaust valves and valve components, parts of steam or gas turbines such as wheels, buckets, bolts, combustion. liners and tail cone assemblies, the metal industry has made use of alloys containing many different alloying elements and having different mechanical properties. Considerable emphasis has been placed in obtaining alloys which are suitable for use at higher and higher temperatures to increase the efiiciency of turbines and the like while at the same-time conserving strategic and critical alloying elements. Both price and mechanical properties have been considered in designing the various parts of steam and gas turbines; Most of the foregoing problems have resulted in the selection of alloys having a practical optimumof the foregoing requirements. However, difiiculties have been encountered in that a number of these alloys while having the required mechanical properties for use at elevated temperatures prove to be f difficult to fabricate or contain high amounts of strategic alloying elements or were exceedingly costly from the standpoint of alloy ingredients and fabrication. An object of this invention is to provide an age hard- 2,824,798 Patented renjzs, 195s With respect to the general range of alloying .elements set forth hereinabove in Table I, each of the alloying elements in the range given exerts a definite influence on' the alloy of this invention. Carbon is ncessary for providing sufficient "strength and hardness in the alloy and to maintain the alloy substantially austenitic throughout the temperature range of the intended use. Alloys having carbon contentsbelow about 0.60% do no t appear to have suflicient strength or hardness whereas alloys with a carbo'ncontent in excess of 1.0% 'while being sufli-' ciently hard have reduced st'reng'thand tend to become very diflicult tofabricate. Manganese .is essential to maintain a stable austenitic structure within the alloy and at least-8.0% manganese is needed. Manganese contents in excess of 15.0% do not appear to substantially contribute to the austenitic stability'and have ,theadve'rse effect of decreasing the strength of the alloy. Silicon IS,

an essential-element and between about 0.25% -and,l.25%

is necessary. While silicon functions :todebiridize; the al-1 -loy and contributes to the formation of ferrite, it has .only a slight efiectnpon the rupture strength. Room temperature hardnes'ses .are practically unaffected by variaenable austenitic alloy having high strength and hardness and which is substantially devoid of strategic, critical or expensive alloying elements.

,Another object ofthis invention is to provide an age hardenable austenitic alloy suitable for use at temperatures of up to 1600" F., said alloy being readily reproducible by standard steel making processes and easily fabricated into the desired components.

A more specific object of this invention is to provide an age hardenable austenitic alloy containing carbon, manganese, silicon, chromium, molybdenum in predetermined amounts and which is devoid of strategic, critical and expensive alloying elements, possesses high strength andhardness and is suitable for use at elevated temperatures of up to 1600 F.

Other objects of this invention will become apparent when read in conjunction with the following description.

This invention in its broadest sense contemplates an. alloy comprising from about 0.60% to 1.0% carbon, from about 8.0% to about 15.0% manganese, from about 0.25% to about 1.25% silicon, from about 9.0% to about 15.0% chromium, from about 1.5% to about 4.0% molybdenum and the balance substantially all iron with not more than 1.0% of incidental impurities such ascopper, nickel, cobalt, phosphorus, sulfur and other imtions in silicon content; however, hardnesses when measured at 1 400 F. indicate that increasing the silicon con- ;tent produces a-- corresponding increase in the hardness.

Chromium is a necessary element to provide a suflicient degree of oxidation and corrosion resistance. At least 9.0%. chromium ismnecessary for adequate protection whereas chromium contents in excess of 15.0% have very poor strength characteristics. Chromium has little effect on the hardness of thesealloys. Molybdenum is used primarily to increase the strength and hardness of the alloy. At least 1.5% molybdenum is necessary in order to obtain suflicient strength and hardness. Molybdenum contents in excess of 4% while exhibiting suflicient hard- 'ness show a decided decrease in the strength such as to detrimentally affect the alloy to an undesirable degree. The balan'ce'of the alloy is substantially all iron with incidental impurities.

Within the general range of composition set forth hereinbefore in Table I the test data indicate that there is a preferred range within which the alloying elements may be varied in order to obtain the optimum combination of properties within the alloy of this invention as will be more clearly set' forth hereinafter. It has been found that where the. carbon content is maintained within the range between 0.70% and 0.85% the optimum combination of strength consistent with good ductility, hardness and austenite stability is obtained. Manganese also exhibits an optimum range in that the optimum combination of strength, hardness, ductility and austenite stability is exhibited where the manganese content is maintained within the range between 10.0% and 14.0%. As was stated hereinbefore silicon is used primarily for deoxidation purposes an d. its effect upon the alloys of this invention indicates that it is usable throughout the range between 0.25 and 1.25%, there being no. optimum range.obsetved. .Sincechromium is a strong ferritizing element, its use must be limited to provide the optimum combination between oxidation" and corrosion :resistance consistent with good ductility, strength and hardness;

For this reason it has been observed that chromium contents within the range between 9.0% and 12.0% are 1 'quite satisfactory for thepurposesofrthis invention. The

7 preferred range of rriolvbdenurn is within the range be-,

tween 2.0% and'3.5%, it being noted that the optimum combination of strength, hardness and ductility has been observed within -the range between 2.0% and 3.5%.

T i is P t sa d the. sfi t m lyb e um 1 .11 the. strength whether the criteria of the time to produce 'ruptureor theitime tofprodiice"l'%,totalstrain isusedas the-basis of evainating t-he-strength.

The alloy"; of 'this' invention is carbon elect-rode electric-arc furnacemelting process, the details Eofrwhich are -:well-: known in the' artand wvill 7 therefone not be described. fit is suflicient=to say thata heat-having a composition wwithin ithe ilimits set-forth hereinbfore :in Table ;I 'iisilimelted and cast into ingots which can fthen'i bev :readily .-fab :icated. iil'ltO the desired semirfi nished mill zpnoduct day .any' ;-:of the well-ilcnown 'processes,' for examplqtor'ging, pressing, r0lling,;textrud inggand theQlilc. The alloy in the fol-mot asemi-finished mill ,prodnctxmay :then lee-fabricated into sa'the desired en'ce maybe had to Tabie ii which'co'ntains the'analy'sis of amumbertdfheats mad'eaancl tested, the compositions of which are both within -=anrloutside-of the :scope of the alloy of thisinvention."

r p I 'ad'efin'anynofthe.

well-knownsteel making -metls1ocls, for -example;-by the either 1o;000'p. s;;1:;0r 15,0130s. 1.:at -aten1peratyre of '1500" 'F; The time to produce 1%total,S .3in, and 5 the "time to produce "rnpture- 11nderthese conditions-,0:

Ta'ble zl. "Anal sis of teas investigated The ,alloys of Tablelll were ngadeland tested in :order j A to show the lefictofseachlof the ialloying elements o n'. 1

the, strength ,andhardness properties of ;these :alloys; Reference is 'directed'to Table ;III which contains :IhCTJ'" results ofthe testslperformedon the alloyssof ffahle il,

illustrating the iefiect of a variation of; each gofitheckrnentsjof the alloy of the invention-orrthe stress-rupture characteristics jthei'eof. In .Table 111 both the time :to

rupture and thetime fto:. p"r0d .1oe- 1 ;tota1-strai n'ar.6- V 7 recorded and menses as criteria insevaluating the all0 ys.,

'The' test: used to evaluate athe s tress t upture properties consisted of subjecting leach 1,:al'1oy V to alconsta'nt load of stress and temperature were observed and-ircw idedfi Note that alllof lt'helallovswerein the heat treated con f V -dition when tested; The detailspof the heatstreatment are set forth in 'IableJII. 7

Table Illa-Stressruptmrelprapertics i 7 I'Iest temperature- 500 F. Heat treatment; 2150" 10.1::hr. w,':o.+14 0;1 ;-15 hrs,?A;:"C]

strss-isooo si. f 1 ;-.s ,ttess-+10.000;p,:s.-1.

-HeatN0. Rnptme' 1%Tota1j ineasqr-lnn mi immoral 11mins 7 7 Time Strain Elong. j Area "Time 5 Strain Elong.- ,Area 7 1 (Hrs.) Time 1 (Percent) (Percent) (-'Hrs.') a Time -(-Percent) (Percenty (Hrs) s- J A. EFFECETOF'CARBON 305 10 1315 17's 1,230 1490 1 6.8 101715 1510 145 7 10. p 13.1 2,305. 7.60 7-1.0 5.9 V 463 11a 10m 10.4 1,853 045 11:2 9.3, 191 7.7 11.0 71953 145,, 113,71 1139 B. EFFECT or MANGANESE 286 5 70v 5.2 13.5 1,236. 410; -17 463 113 10:1 -'10,-4 31,853 245* 0.3 '349 105 4.5, ,1 0.1 1 1, 501 1390i. 4;; 5 s15 91 1 8.01 111 1.1, 171 240 5.10 7

cinnamon srLIooN 403 .105 112.7. 44.1. 1, 007. 500-1 5; 403 113 10:? 110.4" 1,8533 s45 112' 217 105: 1,008: 7 1:7: 325 13 p 6.6 5 as 11,573 ;620; ,913

DIEFFE'CT'OF onnQmrpm I '348' 315: 1.52 335'? 1518 05 :463 113 '1011 :4 1,853

E. E EE TQF MOLEGBDENUM, .03 1101 10 101 0123- 2. 26. 403 113 10.7 ass, yum, 3.11 394 -03 1300 11.4 4.7:; 139 -70 3.8 330 eg-s From the test results recorded in Table III for alloys 6-631, G-630, 6-688 and 6-632 it is seen that in general as the carbon content increases from 0.59% the strength increases to a maximum at about 0.70% to 0.85% carbon and then decreases as the carbon content is further increased to 1.10%. Where the alloys are tested at 1500 F. under an applied stress of l0,000 p. s. i. as well as 15,000 p. s. i. it is found that the optimum strength occurs where the carbon content is in the range between 0.70% and 0.85% as clearly illustrated when both criteria, that is, the time to produce rupture and the time to produce 1% total strain, are compared. The ductility as measured by the elongation and reduction of area is excellent within this range. It will be observed that when the carbon content is increased from abgut 0.85% for alloy 6-688 to about 1.10% for alloy 6-632 that there is a definite decrease in the strength of this alloy. It is for this reason that it is preferred to maintain the carbon content at not greater than 1% maximum. Carbon contents below about 0.60% do not appear to have sufiicient strength for the purposes of the alloys of this invention. Thus from the considerations of strength and ductility, it is apparent that the alloy of this invention must have a carbon content within the range between 0.60% and 1.0% and preferably within the range between about 0.70% and 0.85% for the optimum in strength and ductility.

The efiect of manganese on the alloys of this invention is demonstrated by referring to Subsection B of Table III and the test data recorded therein for alloys 6-633, 6-634, 6-688, 6-635 and G-636. It is seen that increasing the manganese content from 4.0% in alloy 6-633 to 16.72% in alloy 6-636 produces a corresponding increase in both the time to produce rupture and the time to produce 1% total strain when these alloys are tested at 1500 F. and under a stress of 15,000 p. s. i. Thus it is seen that the time to produce rupture isincreased from 155 hours for alloy 6-633 containing 4.0% manganese, up to a maximum of 463 hours for alloy 6-688 containing 9.59% manganese and back to 315 hours for alloy 6-638 containing 16.72% manganese. Corresponding behavior is noted when the data of the criteria of 1% total strain is increased from 125 hours to a maximum of 155 hours for alloy G-688 containing 9.59% manganese. I

Further increase in the manganese of up to 16.72% for alloy G-636 decreased the time to produce 1% total strain to 97 hours. It is thus apparent that manganese contents in excess of about 15% do not appear to be advantageous to the alloy of this invention. The optimum manganese content is in the rangebetween 10.0% and 14.0% from the standpoint of both strength and ductility. Correspondingly similar results were obtained where the tests were conducted at 1500 F. and under a stress of 10,000 p. s. i. Since manganese does not contribute substantially to the strength of the alloy of this invention but is used predominantly for maintaining a completely austenitic structure, it is sufiicient that at least about 8% manganese is used whereas the manganese content should not exceed about 15%. Optimum strength and ductility occur when the manganese content is within the-range between 10.0% and 14.0%.

Referring now to Subsection C of Table III and in particular to alloys' 6-637, 6-688, G-638 and 6-639, it is seen that increasing the silicon contentfrom 0.28% to 1.02% has little effect upon the rupture strength of these alloys. This is especially borne out by the fact that for he alloys e, a 1 a an tt st 1.5. 009.

ass-resa p. s. i. that the only variation in the time to produce 1% total strain was from 105 to about 3 hours as the silicon content was increased .from 0.28% to 1.02%. Since the primary function of silicon is for the purposes of deoxidation and hardness, it is sufiicient that the strength is not detrimentally alfected throughout the intended range of use. Silicon also contributes to the oxidation resistance of these alloys.

The eliect of chromium on the stress rupture properties of these alloys is demonstrated by reference to Subsection D of Table 111 and in particular to alloys 6-640, 6-688 and G-641. These data show that chromium contents of 8.99%, l2.05% and 14.9% produce corresponding rupture times of 348, 364 and 142 hours, respectively, when the alloys are tested at 1500 F. and a stress of 15,000 p. s. i. Similarbehavior is also noted where the stress is reduced to 10,000 p. s. i. Chromium contents below,

about 9.0% do not afford a suflicient resistance to oxidacontents in excess of about 15% detrimentally aifect the,

strength of the alloy so as to make ,it impractical for use. The optimum combination of strength, ductility, corrosion and oxidation resistance appears to occur where the chromium content is within the range between 9.0% and about 12.0%. This is substantiated where the tests were conducted under a stress of 10,000 p. s. i. as well as 15,000 p. s. i., the temperature in both instances being 1500 F.

Referring now to Subsection E of Table III, the efiect of molybdenum is apparent from the test results recorded therein for alloys 6-642, 6-688, 6-643 and 6-644.

"these data illustrate the fact that increasing the molyb denum from about 0.03% up to 4.78% produces a correspending increase in the time to produce rupture of from 101 hours for alloy 6-642 up to a maximum of 463 hours for alloy 6-688 and thereafter decreasing such rupture" time to 139 hours for alloy 6644, the alloys being tested.

at1500 F. and under a stress of 15,000 p. s. i. Similar results were obtained when these alloys were tested at 1500 F. and a stress of 10,000 p. s. i. When the data for the time to produce 1% total strain is examined for the same alloy, it is seen that the maximum time to produce 1% total strain occurs at a somewhat higher molybdenum content than the molybdenum-content necessary for maximum time to produce rupture. Thus the 'maximum rupture life is obtained when the molybdenum content is between about 1.5% and 2.5%, whereas the 1% total strain times are highest for the alloys containing between about 2% and 4% molybdenum; lt is thus preferred to maintain the molybdenum content between 2.0% and 3.5% to obtain tne optimum'combination of strength consistent with good ductility.

From the foregoing, the efiect of each of the alloying elements on the stress rupture properties of these alloys is apparent. In all cases, with the exception of silicon,

increasing the various alloying elements within the ranges given produces a maximum value of strength which occurs within the preferred range of alloying elements for the alloy of this invention.

The alloys of this invention while having outstanding i strength at 1500 F. when tested at stresses of both s liable 11 .-H ar zin ess, properties lieatnienti i g'llreatjmeiltr ::2,160 F.-1nr; 2,150F.1 r.- VHQ.+1:4U01F. V 1 near 4;; "Heat'No. 1 g 7V V l n i 4 Room -1'.40V0 Board; l.4il0 F. "Temp. "(BHNI Temu (BEN) .(BHN) t y 1 1V :.A.EFEEQTJQREQAZRBON 1' SILICON V 7255 142; rats 164 DQEFFECT FOHROMIUM a. armor o1 MOLY-BDENUM V241 121 332' 137 25s- 142 7 364 -164 262-. 204- 7 3s? 7 us 269' 195 387 17s Referring now to alloys 6-631, 6-630, G-688 and 6-632 of Subsection 'A of'TableTV, it'isseen that in generalthe hardness increases as the carbon content is increased -from-ab ut"0,59% to about 1.10%. Between about 0.7% and about 0.9% there is little difference in the hardness of. the alloy in both the solution treated and the :solutionheated plus agedcondition, the hardnesses being measured bothat room temperature and-at-140Q" F. 1 While the, values ofsthe hardness are ,greater where,

the alloys contain greaterthan 0.90% carbon, it will be noted as reported in ,Table 1H thattheistrength greatly decreases. Thus the optimum combination of strength, ductility and hardness is exhibited in these alloys when the carbon content: is in the range between- O;70% and 0.85 Alloys with 'a carbon content below about 0.60%

la'and th solu i nheattreatedand aged condition, 'itris seen that there' is :a general "increase in the hardness as have inferior hardness and strength whereas alloys having a carbon content in excess of 1% have good 'hardness but poortstrength. V V 4 Alloys 6-633, 6-634, 6-688, G-635 and G-636 of Subsection B of Table IVillustrate the effect of manganese on thehardness properties. In generalithe hardness tends to increase .asithe manganese content-is increased from ness in the solution heat treated and agedrconditions.

Metallographic examination indicates that the low manganese alloys contain martensite which is a transformatimum hardness :appearssto occur between .10,0.%.-;and V l4l0% '-.whi chi is ,the samerange for optimum--strength=- and ductility as ortedrin .Table III. 1

1 400, iEsjrar-e compared gboth inithe solution heat treated the silicon 'content-tis increased from 1.028% .to 1.02%

The hardness exhibited throughout the range of silicon tested is siiflicientlyrhigh .that anyyalue inthe whole range can beused, thei'ebeing no .optirnum range ob- 7 served.

Referring now to Subsection '1) of'Ta'ble TV, the efiect of chromium "onfthe"hardness lprqperties is clearly demonstrated by reference to alloys- G-640;G-i688 and G 64 These data *showthat in gen'era-l the'hardness decreases as the chromiur'n content is increased from3899,% to 14.90%

The "decrease 'in 1 the hard'n'ess observed throughout this range isinot appreciable,forthe -alloys in eitherthesolution heat treated or the solution heat treated and aged 7 conditions. The optimum icombinationi of hardness with;

strengthtliesgin the range b.etween19.0% and 12.0%

The mostjefifective, element for increasing the r-hardness is molybdenum, Subsection lifof ',l'abl e lVishows the; testrresultsf or :alloys 6-642, 6-688, 6-643, and 16-644 alid clearly illustrates that increasing the molybdenum content from 0.03% to 4.78% has produced 'a corre spending increase in the BHN' hardness of from 241 to 269 andfrom'332 tof'387'fin the solution heat treated and the solution'heat'treatedanth aged"conditions, respec-T .tively, when measured at roomtemperatui'e and from" 12 1 idarnaximumof 204 and'from l37to 178*1'n the solution heat treated 1 and the solution heat treated and aged conditions, respectiyelwwhen measured -at- 1400' F; 7 Both thegso'lution heat treated and the solution heatttreat- .1;

ed ;and;aged1liardnesses vary. slightly whenthe molybdenum' conten't is increased from about 3.117% to 418%.. V The optimumcombination ofjpropertitasflhatis, strength, ductility andv hardness, is obtained where the molybdenum 7 content does not exceed .3.5 v;

hardness properties it isielearlysen that theoptimum combination of b'oth'streng'th 'and "hardness occurs when i "the alloying elements are withinnthe preferred range set forth hereinbefore in Table "I. Satisfactory results are also -obtainedas long" as the alloying elements are main-f tained Within thegeneral-range as 'set forth hereinbefore in Table I. However,: if the alloying elements are per mitted .to yaryoutsideof the-general range, the strength, 1

' ductility or'hardness areidetrimentally affected so :.that' tion product of unstable austenite. Thus, it is, clearly established that a minimum of 8% manganese must be maintained within the alloysofthis'invention in'or'der,

7 {sinc the alloy 'of 'thi's austenitic itmustibeha'fdenedby an aging heat'treafi merit in order-to'obtainthe full benefit of itsprope rtie's'. This heat-treatment consists of a solutionjrheat treatment the alloyisnot suitable; for'its-intended use at temperatures ofupto 1600;.F. V g H invention is, completely at a temperature withinthe range between'about l950i Fj and 2200 F. for a time period of about to 8f hours depending upon tile-section thickness of the'alloy or article being-heattreated. Penman-applications" the" 'rnate rial will heofi suficienfly smalhsize solthat heating at a'temperature of about 2150 F. for about l'ihourwill T lbe sufilcient to place all. of the hardening elements in: solution in the alloy.f After the alloy is heated for a suflicient time within the temperature range set forth hereinbefore, it is rapidly cooled by quenching, usually into water. If the size of the article made from the alloy of this invention is sufiiciently small in section a less severe quenching medium may be used, for example, air or oil. The solution heat treatment and quenching leaves the alloy article in a soft ductile condition where it can be readily formed and/or machined. The alloy or article is then subjected to an aging treatment at a temperature in the range between 1300 F. and 1600 F. for a time period ranging between 8 and 40 hours depending upon the size. For most applications, an aging time of from 3 to 16 hours will be sufiicient. Thereafter, the alloy is air cooled to room temperature. Excellent results have been obtained where the alloy or article made therefrom is solution heat treated at 2150 F. for about 1 hour, water quenched and thereafter aged at a temperature of about 1400" F. for about 16 hours and thereafter air cooled.

The alloy of this invention is especially useful in applications such as valves and valve components. Where the alloy is formed into an article for use in a highly corrosive atmosphere, for example, an exhaust valve or valve component, it may be desirable to provide the article with a protective surface coating to prolong the life of the article. However, where only a moderate corrosive and oxidation environment is encountered, no protective coating is needed or desired.

There are no special skills required in producing, fabricating or heat treating the alloy of this invention. All equipment used is standard and in use in most steel plants today. In particular, the alloy is exceptionally well suited for use at temperatures of up to 1600 F., readily adapts itself to any fabrication operations, is inexpensive from the standpoint of alloying elements used, fabrication procedures employed and heat treatment operations necessary for obtaining optimum properties, and is devoid of strategic, critical and costly alloying elements.

We claim:

1. An austenitic age hardenable alloy suitable for use at temperatures of up to 1600" F., consisting of from 0.60% to 1.0% carbon, from 8.0% to 15.0% manganese, from 0.25% to 1.25% silicon, from 9.0% to 15.0% chromium, from 1.5% to 4.0% molybdenum and the balance substantially iron with incidental impurities.

2. An austenitic age hardenable alloy suitable for use at temperatures of up to 1600 F., consisting of from 0.70% to 0.85% carbon, from 10.0% to 14.0% manganese, from 0.25% to 1.25% silicon, from 9.0% to 12.0% chromium, from 2.0% to 3.5% molybdenum and the balance substantially iron with incidental impurities.

3. An austenitic age hardenable alloy suitable for use at temperatures of up to 1600 F., consisting of about.

0.85% carbon, about 9.59% manganese, about 0.50% silicon, about 12.05% chromium, about 2.26% molybdenum and the balance iron with incidental impurities.

4. As an article of manufacture for use at elevated temperatures of up to 1600 F., an alloy consisting of from 0.60% to 1.0% carbon, from 8.0% to 15.0% manganese, from 0.25% to 1.25% silicon, from 9.0% to 15.0% chromium, from 1.5% to 4.0% molybdenum and the balance iron with incidental impurities, the alloy being formed to the predetermined shape of the article and being in the solution treated condition resulting from quenching the alloy from a temperature in the range between 1900" F. and 2200 F.

5. As an article of manufacture for use at elevated temperatures of up to 1600 F., an alloy consisting of from 0.60% to 1.0% carbon, from 8.0% to 15.0% manganese, from 0.25% to 1.25% silicon, from 9.0% to 15.0% chromium, from 1.5% to 4.0% molybdenum and the balance iron with incidental impurities, the alloy being formed to the predetermined shape of the article and being in the age hardened condition resulting from quenching the alloy from a temperature in the range between 1900 F. and 2200 F., and aging the alloy at a temperature in the range between 1300 F. and 1600 F. for a time period ranging between 8 and 40 hours and thereafter air cooling to room temperature.

No references cited. 

5. AS AN ARTICLE OF MANUFACTURE FOR USE AT ELEVATED TEMPERATURES OF UP TO 1600* F., AN ALLOY CONSISTING OF FROM 0.60% TO 1.0% CARBON, FROM 8.0% TO 15.0% MANGANESE, FROPM 0.25% TO 1.25% SILICON, FROM 9.0 TO 15.0% CHROMIUM, FROM 1.5% TO 4.0% MOLYBDENUM AND THE BALANCE IRON WITH INCIDENTAL IMPURUTIES, THE ALLOY BEING FORMED TO THE PREDETERMINED SHAPE OF THE ARTICLE AND BEING IN THE AGE HARDENED CONDITION REESULTING FROM QUENCHING THE ALLOY FROM A TEMPERATURE IN THE RANGE BETWEEN 1900* F. AND 2200* F., AND AGING THE ALLOY AT A TEMPERATURE IN THE RANGE BETWEEN 1300* F. AND 1633* F. FOR A TIMER PERIOD RANGING BETWEEN 8 AND 40 HOURS AND THEREAFTER AIR COOLING TO ROOM TEMPERATURE. 